Conventional visible light and near infrared light imaging devices have a configuration in which each of pixels can detect one of three colors of different wavelengths in visible light and near infrared light and such pixels are arranged in a mixed manner in the same imaging element, for example a CCD or CMOS imaging element (e.g. see FIG. 2 of Japanese Patent Laid-Open Application No. 2002-142228). For example, as shown in FIG. 10, Mg (magenta) color detectors 81, Ye (yellow) color detectors 82, Cy (cyan) color detectors 83, and near infrared light detectors 84 are arranged on an imaging element like amosaic so that the combination repeats lengthwise and crosswise. Here, one detector corresponds to just one pixel.
Incidentally, a bare pixel has a property of detecting visible light and near infrared light of 400 nm to 1000 nm. And an Mg color detector 81 is composed of a pixel covered with a bandpass filter for the region of the Mg color. The Ye color detectors 82 and Cy color detectors 83 are also composed of pixels covered with bandpass filters which derive the regions of their respective wavelengths. Bare pixels are arranged as the near infrared light detectors 84. Properly speaking, the visible light region requires to be cut off. But it need not be cut off because obtaining luminance information will suffice. The above configuration allows imaging pictures of both visible light and near infrared light.
In one of previously-suggested visible light imaging devices, three photodiodes are formed on a silicon substrate at different depths from the surface. This conventional device uses absorbance differences in silicon among three kinds of light of different wavelengths in the visible light region, e.g. blue, green, and red, to detect the three kinds of light of different wavelengths (e.g. see FIGS. 5 and 6 of Published Japanese Translation of PCT International Application (Tokuhyo) No. 2002-513145). In this visible light imaging device, three photodiodes of different depths can be arranged in one pixel. This prior art can therefore enable higher color resolution and prevent degradation of image quality caused by false color, in comparison with an art in which one pixel detects one color.
FIG. 11 shows the structure of a pixel formed on an imaging element. An n-type doped region 92 of an n-type semiconductor is formed on a p-type semiconductor substrate 91. Similarly, a p-type doped region 93 is formed thereon. Moreover, an n-type doped region 94 is formed thereon. In this way, a three-layer semiconductor structure (triple-well structure) is made on the substrate. Since each pn interface region can function as a photosensor, three photosensors can be made up in total.
Now, as described in FIG. 5 of Published Japanese Translation of PCT International Application (Tokuhyo) No. 2002-513145 as reference data, blue light is absorbed at a depth of 0.2 μm in a silicon layer. Green is absorbed at 0.6 μm, and red is not absorbed until it reaches a depth of 2 μm. Accordingly, depths of interfaces, which work as photosensors for each light, are set to 0.2 μm, 0.6 μm, and 2 μm respectively. Three colors (three wavelength bands) of light can thus be taken out almost spectrally. Then, an electric current generated by blue light is detected by a current detector 95. Similarly, green light is detected by a current detector 96, and red light is detected by a current detector 97. In this way, colors can be extracted numerically. Of course, it is actually impossible to separate light completely. But a degree of mixing of each color is known (or can be measured) in advance, so corrections can be made.
The above-mentioned three-layer semiconductor doping structure allows one pixel to take out three colors of visible light and can realize a visible light imaging device with significantly improved color resolution.
However, in conventional visible light and near infrared light imaging devices such as the one described in the above-mentioned Japanese Patent Laid-Open Application No. 2002-142228, individual pixels can only detect one color (limited wavelength region). Other colors and luminance information need to be complemented with information from adjacent pixels. Because of this, color resolution and luminance resolution decrease, and false color, which is not the original color, appears.
In addition, an optical system comprising optical lenses commonly provides an image of a subject on the surface of an imaging element. Since focuses fundamentally differ depending on wavelengths, chromatic aberration occurs in imaged pictures (when one color comes into focus, the others go out of focus). So, as shown in FIG. 12, for example, an optical system having a property of an achromatic lens is commonly provided so that an imaging element is within the depth of focus (color shift is negligible within the range) with regard to magenta, yellow, and cyan in the visible light region, and the optical system therefore focuses.
However, there is a limit to achromatism. It is difficult to make a perfect achromatic optical system for a very wide range of wavelengths from the visible light region to the near infrared light region (from 400 nm to 1000 nm). Even if it could be made, such optical system would be very expensive, so that it is difficult to use in current common industries (it is impossible to match all the focus points of visible light and near infrared light to each other). For this reason, when visible light and infrared light are detected on the same photosensitive surface, and if focused pictures are obtained in the visible light region, only out-of-focus pictures can be obtained in the near infrared light region. In other words, if an imaging element whose configuration is as described above were designed on paper, the optical system would be difficult to realize and it is highly likely that such an element could not be put in practical use.
Moreover, conventional visible light imaging devices such as described in Published Japanese Translation of PCT International Application (Tokuhyo) No. 2002-513145 are confined to three-layer semiconductor doping structure and are configured to detect visible light. For that reason, when three primary colors of visible light are to be detected, near infrared light cannot be detected. Suppose that the depth of a photodiode is readjusted such that visible light of two kinds of wavelength regions and near infrared light, a total of three kinds of light, can be detected. However, as described above, it is impossible to make a perfect achromatic optical system for a very wide range of wavelengths from the visible light region to the near infrared light region (from 400 nm to 1000 nm). So if visible light images are to be sharpened, near infrared light images go out of focus. After all, modification of an imaging element alone is insufficient for imaging clearly from visible light to near infrared light.